The present invention relates to a compound semiconductor thin-film solar cell having an n-type buffer layer for heterojunction with a light absorbing layer and a method of fabricating the same compound semiconductor thin-film solar cell.
FIG. 1 shows a basic structure of a thin-film solar cell produced from a general compound semiconductor, which comprises a SLG (soda lime glass) substrate 1 on which a back molybdenum (Mo) electrode layer (positive electrode) 2, a p-type light absorbing layer 5, an n-type heterojunction buffer layer 6 and a transparent electrode layer (negative electrode) 7 are subsequently formed in the described order.
In the thin-film compound semiconductor solar cell, the light absorbing layer 4 is made in the form of a CIGS (Copper-Indium-Gallium-Selenium) thin film made of Cu (In+Ga) Se2 of I-III-VI2 group based on Cu, (In, Ga), Se, which possesses high power conversion efficiency exceeding 18%.
U.S. Pat. No. 4,611,091 discloses a method of forming a heterojunction buffer layer most suited to a light absorbing layer of CIS by chemically growing a thin film of CdS representing a compound semiconductor of II-VI group from a solution.
Japanese Laying-Open Patent Publication No. H-8-330614 describes a heterojunction buffer layer of Zn which does not contain harmful metal such as cadmium and possesses high power conversion efficiency.
The above-described conventional compound thin film solar cells involve such a common problem that a defect easily occur in the junction between a p-type semiconductor light absorbing layer and an n-type semiconductor buffer layer because two layers quite differ from each other by their chemical compositions.
While the light absorbing layer is dipped in the solution, two processes of diffusing Zn element into the light absorbing layer and forming a ZnS film concurrently take place, which may easily cause variations in power conversion efficiency of the product from the crystallinity and surface conditions of the light absorbing layer.
To obtain a heterojunction suitable to the light absorbing layer of CIGS thin film, a buffer layer of InS (InS, InO, InOH) is formed by a CBD method that has been developed to attain the uniformity of composition and reproducibility of the product (see reference “Solar Energy Materials & Solar Cells” 69, 2001, pp. 131-137).
However, the formation of a buffer layer of InS group by the Chemical Bath Deposition (CBD) method for forming a heterojunction suited to the light absorbing layer of CIGS thin film still involves a problem that the buffer layer of InS has a small band gap and generally is hard to pass light of short wavelengths. Namely, it cannot result in high Jsc.